Electrorheological materials are fluid compositions that exhibit substantial changes in rheological properties in the presence of an electric field. Electrorheological materials typically consist of (1) a carrier fluid, (2) a particle component, (3) an activator, and (4) a surfactant. The surfactant of the electrorheological material is utilized to disperse the particle component within the carrier fluid while the activator is utilized to impart electroactivity to the particle component. In the presence of an electric field, the particle component becomes organized so as to increase the apparent viscosity or flow resistance of the overall fluid. Therefore, by manipulating the electric field, one can selectively change the apparent viscosity or flow resistance of an electrorheological material to achieve desired results in various known devices and applications.
Over the years, many different types of electrorheological materials have been developed that are based on numerous types of particle components. These previously developed electrorheological materials may be utilized in various devices, including dampers designed for controlling vibration of a system in either an on/off or continuously variable manner. In many instances, an electrorheological material can be selected in order to provide specific performance characteristics in the particular device or application selected. For example, in a device where it is necessary or desirable to see the inner workings of a device with the human eye, an optically transparent electrorheological material will be selected such as that disclosed in U.S. Pat. No. 5,075,021 entitled "Optically Transparent Electrorheological Fluid."
In certain other devices or applications where the electrorheological material is readily visible to an outside observer, it may be desirable to utilize an electrorheological material that has aesthetic shades of color. In fact, several electrorheological materials have been previously mentioned in the patent literature that utilize dyes or pigments as electrorheological materials. For example, U.S. Pat. No. 3,484,162 describes an electroviscous recording device wherein an electrorheological material is used to create images on a substrate by releasing the material onto the substrate in response to radiation from a lamp that energizes a photoconductive cell to short circuit an electric field that maintains the material in a viscous state. Since the electrorheological materials utilized in the device must have a dark or readily visible color, carbon black is employed as the particle component or a suitable dye or pigment is added to the electrorheological material. U.S. Pat. No. 3,484,162 discloses, as a specific example, a red electroviscous fluid containing Acetamine Rubine B and paraffin oil.
U.S. Pat. No. 3,553,708 discloses another electroviscous recording device wherein materials that exhibit changes in electroactivity in the presence of actinic radiation or light and an electric field are utilized in the electrorheological material. A constant electric field is applied to the electrorheological material and a lamp is utilized to release the electrorheological material at selected intervals in response to changes in radiation. The electrorheological materials utilize various dyes and pigments including phthalocyanine-type compounds such as copper phthalocyanine. The carrier liquid of the materials may be any dielectric liquid such as mineral oil, chlorinated hydrocarbons, fluorinated hydrocarbons, etc.
U.S. Pat. No. 4,687,589 describes an electrorheological material that utilizes as the particle component a substantially anhydrous electronic conductor such as an organic semiconductor comprised of unsaturated, fused polycyclic systems containing conjugated .pi.-bonds. Specific examples of particle components include phthalocyanine-type compounds such as copper phthalocyanine, violanthrone B, porphin or azaporphin systems, poly(acene-quinone) polymers, and polymeric Schiff's Bases.
Previously developed electrorheological materials utilizing dyes and pigments such as those described above frequently suffer from various disadvantages. For example, the responsiveness of many dyes and pigments to an electric field is relatively sluggish so that the particular electrorheological material may exhibit an undesirable delay after exposure to an electric field. Additionally, electrorheological materials such as those disclosed in U.S. Pat. No. 3,553,708 require the presence of actinic radiation in addition to exposure to an electric field. Finally, substantially anhydrous electrorheological materials such as those disclosed in U.S. Pat. No. 4,687,589 have been found to require the use of an alternating current (A.C.) electric field to limit particle electrophoresis (the migration of the electrorheological particles towards one of the electrodes) which can interfere with electrorheological activity.
It is desirable in most electrorheological material applications such as on/off and continuously variable vibration damping, visual effects displays or the like, for an electrorheological material to exhibit a response to an electric field on a millisecond time scale. Since many applications do not lend themselves to open exposure to light, it also is desirable for an electrorheological material to exhibit an electrorheological response in the absence of actinic radiation. Due to the high cost and limited availability of A.C. high voltage power supplies, it is furthermore desirable that an electrorheological material be capable of being operated with direct current (D.C.) voltage. A need therefore exists for an electrorheological material which is based on a dye or pigment and which would exhibit a relatively fast response time, would not require the presence of light in order to exhibit an electrorheological effect, and would effectively operate with the application of D.C. voltage.